So here's a list of potential titles - my suggestion would be that a slimmed down version of the list be available to rulers to choose from when it comes to how their realm's "royals" are referred to:
Royal of Keplerstan
Prince of Keplerstan
Dowager King/Queen of Keplerstan
Statesman/Stateswoman of Keplerstan
(Grey?) Eminence of Keplerstan
Senior of Keplerstan
Honoured of Keplerstan